1. Field of the Invention
The present invention is directed to an LED indicator and a driving circuit to drive an LED. More particularly, the present invention is directed to an LED indicator and a driving circuit that can drive an LED with a compensation for a loss in the luminous output of the LED. This invention can find particular application when the LED is utilized in a device such as a traffic signal or another indicating signal.
2. Discussion of the Background
The use of LEDs in indicating devices, such as traffic signals, is known. One drawback with using LEDs in an indicator such as a traffic signal is that luminous output of an LED degrades with both time and increasing temperature. For red LEDs degradation with respect to temperature will typically result in a loss of approximately one percent of intensity of the LED with every one degree Celsius increase in temperature. Conversely, as temperature decreases, intensity of light output from an LED increases. Moreover, LEDs gradually degrade over time, and thus become dimmer as they get older.
Known systems sense temperature at the LED or sense light output at the LED. and utilize the sensed temperature or sensed light output as a feedback to a power supply. Such a system is disclosed in U.S. Pat. No. 5,783,909 to Hochstein. This patent discloses (1) sensing temperature at an LED or sensing intensity output from an LED, (2) feeding back a signal proportional to the sensed temperature or intensity to a power supply, and (3) then increasing or decreasing the average current output by the power supply based on an increase or decrease in temperature in the light output of the LED.
In such a known system, sensing a luminous output of an LED may provide a benefit over sensing a temperature at the LED. Specifically, sensing luminous output of an LED allows compensation for both temperature-induced and age-induced degradation of the luminous output by the LED.
However, providing a photosensor to accurately detect the luminous output of an LED is somewhat problematic.
More particularly, to accurately detect the luminous output of an LED all other external stray light sources, e.g. sunlight, must be disregarded. That is, to provide an accurate feedback signal of a luminous output of an LED a photodetector must only detect the luminous output of the LED and cannot be affected by other forms of stray light, such as sunlight.
A second requirement of a photosensor is that it must gather light from a large enough sample of LEDs to be representative of all the LEDs in the lamp.